1. Field of the Invention
The present invention generally relates to a phase shift mask.
2. Description of the Related Art
Photolithography generally includes the processes of resist coating, exposure, development, etching, and removal of resist. Exposure is a process of causing an exposure apparatus to transfer a mask pattern onto a photosensitive material (resist) coated on a wafer so that a latent image pattern can be formed on the wafer. In exposure, three factors: resolution, overlay accuracy, and throughput are important. Resolution represents a minimum dimension at which transferring a pattern is feasible. Overlay accuracy represents the accuracy in the process of overlaying a pattern on a wafer with another pattern. Throughput represents the number of wafers processed per unit time.
In the manufacturing of a device using photolithography technology, an exposure apparatus causes a projection optical system to project a pattern of a mask onto a wafer. The projection optical system causes diffracted light from the mask pattern to interfere and form an image on the wafer.
The following equation (Rayleigh equation) defines the resolution R of a projection exposure apparatus where λ represents the wavelength of a light source and NA represents a numerical aperture of the projection optical system.R=k1(λ/NA)In the equation, constant k1 is a variable determined according to a development process. In the ordinary exposure, k1 is in a range approximately from 0.5 to 0.7.
Recent highly integrated devices require, in manufacturing, transferring a fine pattern onto a wafer. More specifically, high resolution is required. As apparent from the above-described equation, increasing the numerical aperture (NA) and reducing the wavelength (λ) are effective to realize high resolution. An immersion exposure apparatus includes an internal space filled with liquid (e.g. water) between a final lens of the projection optical system and a resist surface. The immersion exposure apparatus can increase the numerical aperture (NA) and, therefore, can realize high resolution. For example, a recently developed immersion exposure apparatus has an NA value equal to or greater than 1. The refractive index of water is approximately 1.44.
However, when the numerical aperture (NA) is increased, polarization of light influences imaging performances. If light is incident on a wafer at a large angle, imaging performances may change due to a difference in polarization direction of the light. Therefore, as discussed in Japanese Patent Application Laid-Open No. 2006-135346, a conventional technique that is useful in improving imaging performances illuminates a mask with only polarized light.
As discussed in IEEE Transaction On Electron Devices, Vol. ED-29, No. 12, DECEMBER 1982, pp. 1828-1836, a Levenson phase shift mask can improve resolution in image formation of a fine pattern composed of thin lines. Usage of the Levenson phase shift mask is effective to reduce the constant k1. FIG. 21 illustrates an example Levenson phase shift pattern, which includes trenches of a glass substrate disposed between light-blocking portions extending in parallel to each other on one side of the glass substrate. The Levenson phase shift pattern illustrated in FIG. 21 defines a phase difference of 180° (π). The portion defining a phase difference is referred to as a phase shifter or a shifter.
FIG. 22 illustrates an example “small σ illumination” having higher coherency. The “small σ illumination” has a small aperture as inlet of exposure light. The “small σ illumination” can transfer a phase shift mask pattern onto a wafer as illustrated in FIG. 23.
The method using the small σ illumination, as discussed in Japanese Patent Application Laid-Open No. 5-109601, includes setting the direction of polarized light to one direction, setting the direction of a mask pattern to one direction, and illuminating a mask with polarized light useful in improving imaging performances.
The phase shift mask is subjected to a problem generally referred to as “0/π difference.” The “0/π difference” causes a dimensional difference between a normal aperture portion and a trenched aperture portion (phase shift portion) as illustrated in FIG. 21. More specifically, even if two aperture portions have the same dimensions in a plan view, a difference in intensity of light may occur between two aperture portions. Then, a significant amount of dimensional error may occur in the result of exposure.
As discussed in detail in Japanese Patent Application Laid-Open No. 2005-345960, the phase shift mask has a unique structure (referred to “undercut”) illustrated in FIG. 24 that includes a trench of a substrate and an overhang of an absorber (e.g., chrome) protruding from an upper end of the trench. The “undercut” structure illustrated in FIG. 24 can eliminate adverse effects of reflections of light on side walls of a trench and can reduce the effect of a dimensional difference in a result of exposure. Furthermore, the added method provides a dimensional difference between an aperture portion corresponding to phase 0 and an aperture portion corresponding to phase n and, as a result, brings an effect of correcting an error by an amount equivalent to the provided dimensional difference. FIG. 24 illustrates a dimensional difference referred to as “bias” added to a trench.
As discussed in Japanese Patent Application Laid-Open No. 2005-345960, when a pitch is set to 480 nm on a mask (which is equivalent to a pitch of 120 nm, a line width of 60 nm, and a space width of 60 nm on a wafer), the “0/n difference” can be eliminated by setting an overhang having a length of 80 nm and setting a difference of 60 nm (30 nm on each side) as “0/π dimensional difference” on a mask. A desired length of the overhang is equal to or greater than 0.2λ (e.g., equal to or greater than 40 nm when the wavelength of ArF is used).
To obtain an image having a pitch of 90 nm, a line width L of 32 nm, and a space width S of 58 nm, it is desired to use a pattern having a pitch of 360 nm, a line width L of 128 nm, and a space width S of 224 nm on a 4× mask. In this case, if a light-blocking portion (absorber) is configured as a line portion, adding an overhang having a length equal to or greater than 40 nm to an absorber having a width of 128 nm is difficult.
And, to obtain an image having a pitch of 90 nm and a line width L of 32 nm with an exposure apparatus having a wavelength of ArF (193 nm) and NA=1.35, it is possible to perform simulation with respect to image formation of a pattern having a pitch of 360 nm and a line width L of 128 nm on a 4× mask while setting a absorber (chrome) thickness to 103 nm. FIG. 25A illustrates, as a problem caused by the “0/π difference”, a difference in intensity of light between a trench and a non-trench region occurring even if two aperture portions have the same dimension when seen in a plan view. A large dimensional difference or a large image shift may occur when a large difference in intensity of light is caused between a trench and a non-trench region.
The peak difference in intensity of light between a trench and a non-trench region can be estimated by changing the amount of “undercut” and the amount of “bias.” The simulation includes illuminating a mask with transverse electric (TE) polarized light and causing a projection optical system to form an image of a mask pattern on a wafer. The mask has a cross-sectional structure (more specifically, a three-dimensional structure) whose dimension is smaller than the wavelength of illumination light. Thus, the simulation includes accurately reproducing a three-dimensional structure of a mask and obtaining diffracted light by performing electromagnetic field analysis on the reproduced three-dimensional structure of the mask. The illumination used for a phase shift mask is usually a coherent illumination. In the electromagnetic field analysis, the incident wave is perpendicular to the mask.
FIG. 25B illustrates results obtained by the simulation. The amount of “undercut” has a positive value if it increases the width of a trench. The amount of “bias” has a positive value if it increases the width S of an absorber. The reference point is set to a position where the line width and the space width have original values (L=128 nm and S=224 nm). In FIG. 25B, the abscissa axis indicates the amount of “undercut” and the ordinate axis indicates a peak difference (I1−I2) in intensity of light between a peak (I1) at anon-trench region and a peak (I2) at a trench in a best focus state. FIG. 25B illustrates some examples of the peak difference (I1−I2) obtained by changing a combination of “bias” and “undercut.”
As understood from the results illustrated in FIG. 25B, some of the combinations of “bias” and “undercut” can eliminate the peak difference (I2−I1) in intensity of light. However, it is unknown which combination is suitable for solving the “0/π difference” problem. More specifically, it is unknown how to determine the amount of “bias” and the amount of “undercut.”
There are various mask structures (cross-sectional structures) conventionally known as discussed in Japanese Patent Application Laid-Open No. 2005-182031 or in Japanese Patent Application Laid-Open No. 2005-321641. However, it is unknown which structure is an optimum structure for resolving a pitch less than the wavelength of illumination light.
If the half pitch or the width of an absorber becomes smaller than 45 nm, it is difficult for the conventional mask structures to set a sufficient amount of “undercut” that can eliminate the “0/π difference.” It is unknown how to determine the amount of “undercut” as well as the amount of “bias” to eliminate the “0/π difference.”
Furthermore, if a mask structure has dimensions equivalent to or less than the wavelength of illumination light, diffraction efficiency may change depending on the polarization direction according to a three-dimensional structure of the mask. It is unclear whether a change in diffraction efficiency has adverse effects on exposure performances. On the other hand, if a polarized light illumination is used, an exposure apparatus may include an error in the direction of polarized light. Although a deviation from the target polarization direction is a small amount equivalent to approximately ±1% of the entire intensity, CD error may occur due to device differences in respective exposure apparatuses, or polarization control, or polarization direction changes occurring due to birefringence in a glass material.